The present invention relates to an infrared cut-off film which is affixed on, for example, a windowpane of a building or a car mainly in order to cut-off the infrared of sun light.
Conventionally, there is used an functional film having a infrared cut-off function, which film shows optical transmission properties to a light in a visible light region and which film reflects or absorbs a light in a infrared region, mainly for inhibiting the thermal influence of radiated sun light. For example, the functional film adherent to a windowpane of a building or a car reduces heat, even when a solar radiation is directly received through the windowpane. Further, in summer, the increase of room temperature is reduced so that cooling efficiency is increased. Furthermore, it gives an additional effect, e.g., the prevention of shattering of the windowpane in a case of a windowpane break.
The above infrared cut-off film has a multi-layered structure, for example, in which a protective layer is stacked on the front surface of a base film and an infrared cut-off layer and an adhesive layer are stacked on the reverse surface in this order. And, the infrared cut-off film is used by affixing the adhesive layer onto a glass or the like. Conventionally, the infrared cut-off layer is formed on a base film by using an infrared absorbent of imonium, aminium or an anthraquinone-containing compound or an infrared reflecting agent of ZnO, SnO2 or a phthalocyanine-containing pigment as an infrared cut-off agent and forming a layer of the infrared cut-off agent by a vacuum deposition method, a spattering method or a method in which a coating composition obtained by dispersing the infrared cut-off agent in a proper resin and used for a infrared cut-off layer is applied.
However, the conventional infrared cut-off agent is colored to have, for example, a puce or cobalt color. As a result, it has poor transparency of a visible light transmittance of 50% or lower. Otherwise, it cuts off only infrared radiation in a long wavelength region of 1000 nm or more, or 1500 nm or more or it cuts off only infrared radiation in a very narrow range of wavelength region.
Thus, a powder of indium tin oxide (ITO, hereinafter) receives attention as a material for improving the defects of the conventional infrared cut-off agent, and it is actually used.
Conventional infrared cut-off films comprising an ITO powder in an infrared cut-off layer have a hue of blue-green or green in most cases and the conventional infrared cut-off films do not have sufficient transparency. Further, a blue film having a sense of transparency is generally preferred as an infrared cut-off film used by affixing it on a windowpane or the like. Therefore, the conventional infrared cut-off films have not sufficiently satisfied the above requirement.
Therefore, it is an object of the present invention to provide an infrared cut-off film having a hue of blue and sufficient transparency in spite of the use of an ITO powder.
The present invention provides an infrared cut-off film having an infrared cut-off layer in which a powder of indium tin oxide is dispersed,
wherein the powder of indium tin oxide has a minimum value of a diffused-reflection-functional logarithm, logf(Rd), at a light wavelength of 470 nm or lower, which logarithm is measured on the basis of the following equation formulated for a diffused reflection light,
f(Rd)=(1xe2x88x92Rd)2/2Rd=xcex1/S
xe2x80x83in which Rd is a relative reflectance to a standard sample, xcex1 is an absorption coefficient and S is a scattering coefficient,
xe2x80x83and the above minimum value is xe2x88x920.1 or less.